1. Field of the Invention
The present invention relates to transistor devices having a pilot structure.
2. Description of Related Art
Semiconductor devices often include in addition to the main device, a pilot structure or device which is a small scale version of the main device. The main device and pilot device are monolithically formed on the same semiconductor chip and the current through the pilot device is generally proportional to the current through the main device. Accordingly, by measuring the current through the pilot device, a reasonably accurate indication of the current through the main device is provided. Thus, if the current through the pilot device reaches a predetermined level, indicating a dangerously high current through the main device, the main device can be shut off thereby preventing damage to the main device or devices connected to the main device.
To prevent latch up between the main device and the pilot device, two or more spaced channel regions have been formed between the main device and the pilot device. The channel region closest to the main device has typically been connected to the main device and the channel region closest to the pilot device has typically been connected to the pilot device. To control the conductivity of the region between the channel regions, an electrode has been insulatively disposed over this region. The gate electrode has usually been connected to the gate electrodes of the main and pilot devices or alternatively, has been connected to the channel region closest to the main device.
In addition to current sensing functions, it is also desirable to use the pilot device to sense the operating voltage of the main device. However, in voltage sensing applications, a significant voltage difference between the main device and the pilot device can develop which can divert current from the pilot device to the main device through the spaced channel regions. As a consequence, the pilot device may fail to indicate an impending over voltage condition with potentially destructive consequences.